Various methods and apparatus have been devised for measuring characteristics of the motion of an object, such as velocity, estimated distance the object will travel ("carry distance"), spin, momentum, and trajectory. Radar devices have been developed which utilize the Doppler frequency shift to measure the velocity of the moving object. Very briefly, electromagnetic energy, such as microwave radar energy, which is transmitted toward and reflected by a moving object undergoes a frequency shift, the magnitude of which is proportional to the velocity of the object relative to the transmitter. Samples of the transmitted and reflected radiation are mixed and processed to obtain a difference signal having a frequency which is equal to the difference between the transmitted and reflected frequencies, this difference being the Doppler shift. Once the difference frequency has been obtained, the relative velocity of the object can be readily calculated.
Many Doppler radar devices count the number of pulses in the difference signal during a predetermined period of time or "window." If the width of the window (i.e., the period of time) is chosen properly, the number of pulses which are counted will equal the velocity of the object in the desired units (such as miles per hours or kilometers per hour). To determine the width of the window, it is necessary to apply the following formula: EQU f.sub.d =(2f.sub.t v.sub.r)/v.sub.c
where f.sub.d is the Doppler frequency; f.sub.t is the frequency of the transmitted radiation; v.sub.r is the relative velocity of the object; and v.sub.c is the velocity of light in appropriate units. For a transmission frequency f.sub.t of about 10.5 GHz (a typical operating frequency for Doppler radar), f.sub.d equals about 31.3 v.sub.r (in miles per hour). The width of the window is the inverse of 31.3, or about 31.9 milliseconds, and the number of difference frequency pulses counted will give the object's velocity in miles per hour. For example, an object moving 100 miles per hour would produce a signal with a Doppler difference frequency of about 3,130 Hz. The number of pulses in the signal counted during a window having a width of 31.9 milliseconds is about 100, which is the velocity of the object in miles per hour.
A significant disadvantage of pulse counting to obtain the velocity of an object is that signal "drop-outs", noise and other interference may increase or decrease the actual number of pulses counted during the window period, thereby degrading the accuracy of the device. For reasons which are not fully known, a portion of the reflected signal may not be detected, leading to periods during which no pulses are received (drop-out periods). Additionally, noise can introduce false pulses, thereby increasing the number counted. While various filtering techniques have been proposed to reduce the effects of noise, they may not be completely effective and may have little or no ability to offset the effects of signal drop out.
Many Doppler radar devices employ phase lock loop (PLL) circuitry to "lock" onto the difference frequency and to generate a voltage which is proportional to the Doppler frequency. Additionally, an internal oscillator is synchronized with the frequency of the difference signal and provides an output signal at that frequency. The status of the constant voltage output can be used to determine when the PLL has locked onto the moving object (i.e. when the oscillator becomes synchronized with the difference signal). When synchronization occurs, the constant voltage output can be used to initiate the counting of pulses from the oscillator during the predetermined window.
When a PLL Doppler radar device is on, the PLL generates an output signal regardless of whether a lock has not been achieved. This signal, which resembles random noise, can, in some circumstances, make it difficult for a lock to be accomplished. Additionally, such a device typically requires many components and may, in fact, have to be fabricated on several circuit boards. This raises reliability issues related to the quality control of parts and production. It can be appreciated that a relatively high failure rate can result in increased production costs when faulty units are rebuilt, repaired or simply discarded.
Many Doppler radar devices employ a resistive/capacitive (RC) network in order to establish the width of the timing window. Using known equations, the values of the components in the RC network can be calculated to enable a capacitor to charge to a predetermined level, thereby activating or deactivating a counter. Precise and expensive components are necessary to provide a very accurate system; even when precision components are employed, accuracy may suffer due to age, heat and the like. Crystal controlled timing circuits are generally more accurate but may be more expensive than an RC network and may require additional components to produce usable timing pulses.
Both methods of establishing a window width have the common disadvantage of being relatively inflexible when more than one window width is desired. For example, if velocity is desired in units of kilometers per hours rather than miles per hour, the window width should be about 5.1 milliseconds for a transmission frequency of about 10.5 GHz. Consequently, some means, such as additional timing circuitry, must be included to allow the desired units to be selected.
Another application in which it is desirable to select from among different window widths involves the use of a Doppler radar device to calculate the distance an object can be expected to travel. For example, during custom golf club fitting or during golf training and practice, it can be useful to know how far a golf ball will travel ("carry distance"). During golf club fitting, the golfer can try out different sized clubs and clubs from different manufacturers to determine which will enable him or her to consistently hit the farthest. During lessons or practice, a golfer can change his or her grip, stance or swing in order to adjust or maximize the carry distance of the golf ball. It can be appreciated, therefore, that accurate information about the ball's carry distance would be extremely useful. It would also be useful to be able to obtain such information indoors in a relatively small enclosed space.
A further disadvantage to counting pulses during a window period is that the ball may be near or beyond the range of the device before the end of the window, making the device more susceptible to noise and interference and reducing its accuracy. Increasing the range of the device, such as by increasing its power output or the input gain, may be expensive or impractical and may cause the device to be more sensitive to moving objects other than the desired target.
In fact, one of the biggest problems with prior art devices which measure the carry distance of a golf ball is that moving objects other than the desired target often cause inaccurate readings. For instance, if such a device were used at a driving range and someone nearby hit a ball into the path of the transmitted radiation, a prior art device would have no way to distinguish that ball from a ball hit by the user of the device. This could cause the PLL to lock on to the wrong ball or even prevent a lock altogether. In addition, since prior art devices do not include means to isolate only the motion of the appropriate golf ball, these devices are placed beside the teed golf ball rather than behind it, so that the motion of the club head does not affect the readings. Placing the device beside the golfer can reduce the accuracy of a distance measurement based on relative velocity. In addition, from a position beside the golfer, a prior art device may not be able to track a shot hit with a great deal of loft which is outside of the cone of transmitted radiation.
Consequently, it is desirable to provide increased accuracy with a single radar unit having selectable parameters without relying on fixed timing windows and without employing an excessive number of circuit components. It is also desirable for such a device to be faster, more accurate, and less susceptible to noise and drop-outs than existing devices. Finally, it would be advantageous for such a device to be capable of determining whether received data is related to the appropriate moving object, so as to obviate the effects of other moving objects within the unit's range.